It is often desirable to be able to communicate between a number of stations in the same building without installing a dedicated data transmission network. One approach for achieving such a data link is to use the building's existing power line transmission network. Data transmission systems based on existing power line transmission networks are generally referred to as Power Line Carrier (PLC) systems or Power Line Data Transmission (PLDT) systems.
Most of the previous applications for PLDT systems have involved single station to single station communications. Examples of such systems include intercoms and appliance controllers. In some applications, such as appliance controllers, a number of receivers are connected to the power line, but are controlled by a single transmitter. Systems involving multiple transmitters and multiple receivers have not been used in the past because of a number of difficulties involved in coupling the multiple transmitters and receivers (transceivers) to the power transmission line. In particular, previous systems attempting to employ multiple transceivers have had a very limited range and effectiveness because of line coupling difficulties.
A typical AC power transmission line normally has a very low impedance, on the order of one to ten ohms. The low impedance of the AC line at frequencies usable for communications causes significant difficulties with regard to the filtering of the transmitted and received signals. The low line impedance limits the quality factor (Q) of any filter coupled to the line and thus causes significant degradation of the filter rolloff characteristics. Furthermore, the impedance of the AC line varies significantly with time. This variance in the line impedance tends to change the frequency response of the filters which leads to additional coupling difficulties.
Because of the above-mentioned problems associated with the transmission characteristics of AC power transmission lines, ordinary transceiver filters cannot be effectively adapted to PLDT applications. In particular, the transmitter and the receiver portion of a transceiver have different filtering requirements when used for communications over power lines. In general, the filter used for the transmitter should have a very low loss, while providing moderate rejection and a fairly wide bandwidth. The loss of the transmitter filter must be low because the power requirements needed to overcome loss rise very rapidly and thus quickly become unfeasible. Bandwidth and rejection are not particularly important for a transmitter, with the exception of harmonic rejection and rejection of intermodulation signals.
The receiver portion of the transceiver can withstand a fairly high loss factor, on the order of 20 dB, but the rejection should be as high as possible. The filter loss can be high because the signal to noise ratio of the received signal is limited by the noise present on the power line. The bandwidth requirements of the receiver are set by the frequency requirements of the particular modulation scheme being employed.
Previous coupling circuits for connecting transceivers to power transmission lines typically employ parallel tuned tank circuits comprising a capacitor in series with the secondary winding of a transformer. This type of coupling circuit defines a bandpass filter having a relatively low loss and moderate rejection. While this circuit is suitable for use by the transmitter, it does not meet the filtering requirements of the receiver. Systems using a single coupling circuit of this type for filtering the signals of both the transmitter and the receiver have, therefore, have been ineffective for the reasons discussed above.